Measured against the peace-time record of British prime ministers since the end of the First World War, few of whom achieved much, Mr Blair's premiership compares favourably.

Ian Kershaw said there were unrealistic expectations in 1997

Measured against the shambolic record of the last Conservative government, under Mr Major, it compares even better.

But measured against the expectations of 1997 (for the moment leaving Iraq out of the equation), and judged by perceptions of the present state of British society, the 10 years of Mr Blair will surely go down in history as a lost opportunity, a time when much was promised but relatively little attained.

Of course, Blair transformed the Labour Party and presided over three consecutive general election victories, a feat which had eluded every previous Labour leader. But to what purpose?

Undeniably, a decade of sustained prosperity and economic stability, unparalleled in recent times, has been a remarkable achievement, even if Gordon Brown was the main architect of the economic success.

On constitutional matters, devolution in Scotland and Wales has been successful, at least in the short term, though probably it has only fended off rather than excluded a more damaging nationalist challenge (at least in Scotland) to the United Kingdom's integrity.

The expectations of 1997 were quite unrealistic

And in Northern Ireland, though he did not begin the process, Blair's negotiation of the Good Friday Agreement was the real start of major change for the better.

But in the workings of British government, the Blair era has not been beneficial. The use of "spin doctors" and unelected advisers has been harmful to democracy.

Ten years of presidential-style government have eroded the authority of the House of Commons. And effective reform of the House of Lords is still a distant dream.

The expectations of 1997 were quite unrealistic. In the euphoria of finally getting rid of a clapped-out, shoddy Conservative government long past its sell-by date, quite illusory hopes were invested in the new Labour regime.

Deep-seated problems

Blair's own propaganda inflated these hopes by hyped-up claims that his premiership marked the start of a new era. Disillusionment was inevitable.

In reality, the deep-seated structural problems of the health system, education, transport, crime, juvenile disorder, urban decay, and inner-city deprivation were never going to be overcome, whatever the colour of the government, within a decade or so.

Even so, there is remarkably little substantial improvement to show for 10 years of Labour government, particularly in the position of the less well-off in society. Yet what is Labour for, if not above all to serve their interests.

Could Iraq be Mr Blair's Suez?

The introduction of a minimum wage, set at a modest level, was admittedly a step in the right direction.

The government also uses statistics ad nauseam to demonstrate major improvements in health, education, crime reduction, and so on. But few people are persuaded.

To those at the sharp end, the improvements are often scarcely visible. Overall, the feeling is that taxes have gone up while public services have deteriorated.

For many living in the most deprived inner-city areas, there is little sense that the insecurity produced by crime, disorder, vandalism and anti-social behaviour has been much reduced.

In schools and hospitals marginal improvements at best can be seen, and many professionals complain that things are even worse than when Labour took power.

Despite the money poured in, the health service is still in poor shape. In schools, new buildings can provide better conditions. But the stress levels for teachers are enormous. Students now face high fees and indebtedness, though few of their lecturers would acknowledge a significant positive transformation in universities.

Doctors, nurses, teachers, policemen and others suffer, too, more than ever from the oppressive bureaucracy of an unconstrained control culture.

Disappearing idealism

Meanwhile, transport remains a mess, with no integrated planning or joined-up thinking in view. Heads here have been firmly stuck in the sand while the problems mount.

The litany could continue. The biggest indictment overall is that, after ten years of a Labour government, people have little or no sense of greater social fairness.

The idealism built into Labour's social vision has all but disappeared. No wonder there is widespread apathy about party politics, which are as riddled with sleaze as ever they were under the Tories.

Labour now seems to stand for little more than the claim that it can manage the problems of British society a bit better, and a bit more humanely, than can the Conservatives. And even that claim is open to question.

However Blair's domestic achievements are judged, his place in history will be primarily shaped by the Iraq war. Iraq will forever stand out in bold red in the debit column of his time in office. It was an avoidable disaster. And it was a disaster bearing Blair's personal hallmark.

One word can epitomise a premiership for posterity. For Anthony Eden it was "Suez". For Blair, it will be "Iraq".

Before 11 September 2001, Tony Blair was set to go down in history as a second-division prime minister, one of those who stayed in power for a long time but without having any appreciable effect on the story of his times.

Mr Roberts said Blair was a good war leader

He followed the opinions of focus groups and opinion polls, carefully judging every option with the sole criterion of getting New Labour re-elected.

If anything, he was something of a mild wrecker: over devolution, the House of Lords, hunting, and the Lord Chancellorship, he seemed happy to rip up ancient British arrangements without giving much thought as to what would be likely to replace them.

Then came 9/11. Suddenly, everything changed.

'Shoulder to shoulder'

No fewer than 67 Britons died on that day, and Blair showed a side of his personality that had been impossible to discern before: Churchillian leadership.

With his announcement that Britain would stand "shoulder to shoulder" with the United States, and backing that up with sending large numbers of British troops to both Afghanistan and Iraq, he carved a place for himself in the first rank of British premiers since 1900.

Was Blair, like Churchill, in the first rank of prime ministers?

Blair was watching television in Brighton when the third plane hit the Pentagon at 2.43pm (British time), and then put in a short appearance at the Trade Union conference there.

Visibly shaken, he told the delegates: "There have been the most terrible, shocking events in the United States of America in the last hours.

"I am afraid we can only imagine the terror and carnage there and the many, many innocent people who have lost their lives. This mass terrorism is the new evil in our world today."

Later he recalled, in an interview on Boston television: "Sometimes things happen in politics, an event so cataclysmic that, in a curious way, all the doubt is removed.

"From the outset, I really felt very certain as to what had to be said and done."

Blair proved himself an exemplary war leader

This was underlined soon afterwards in his powerful address to the Labour Party conference the next month, in which Blair said of the American people: "We were with you at the first. We will stay with you till the last."

In this he was as good as his word.

Blair had fought wars before, of course. Kosovo and Sierra Leone had seen British troops deployed by him, both successfully and with minimum casualties.

The post-9/11 invasions of Afghanistan and Iraq were to be of an altogether different - and larger - order, although the casualties were also amazingly light.

Blair proved himself an exemplary war leader.

WMD danger

Under the sincere impression that Saddam Hussein of Iraq possessed chemical and biological weapons of mass destruction (WMD) - a view shared by the intelligence services of every major country in the world, as well as by the United Nations' own weapons inspectors - he appreciated the importance of overthrowing the Baathist regime as soon as the Taleban regime was deposed in Afghanistan.

The dangers of WMDs possibly falling into the hands of anti-Western terrorists groups could only be obviated by a full-scale US-led invasion by the Coalition of the Willing, and none proved more willing than Blair.

(The fact that it later transpired that Saddam had most probably destroyed his WMD was entirely immaterial; Blair believed Saddam had had them at the time, not least because he had used them extensively on his own people in the late 1980s.)

Prime ministers...are judged by the One Big Thing that happens during their premierships.

In Britain a vicious and disgraceful campaign began, attempting, on the flimsiest of evidence taken wildly out of context, to accuse Blair of deliberately lying in order to take the country to war.

Its smears and slurs will not stand the test of historical analysis.

The defeat of the enormous Iraqi army in only three weeks in March 2003; the establishment of democracy in Iraq, with a 70% turnout in nationwide elections on 15 December 2005; the stalwart defence of Iraq against an unexpectedly long and violent terrorist insurgency; the loss of only tiny numbers of British troops by any historical or operational standards: all will be credited to Tony Blair by posterity.

Similarly the burning out of Al Qaeda and the Taleban from the most important areas of Afghanistan is another of his fine achievements.

Prime ministers are not judged by posterity on issues to do with transport, health, education, or even - most of them - on economic indicators.

They are judged by the One Big Thing that happens during their premierships.

That is why Neville Chamberlain's Munich Agreement, Anthony Eden's Suez Crisis, Edward Heath's Three-Day Week, and John Major's ERM debacle have left them branded as failures.

Equally, Winston Churchill's Blitz orations, Margaret Thatcher's saving of British capitalism and Tony Blair's vigorous prosecution of the War against Terror will leave them noted by history as highly successful prime ministers.

The verdict of history on all Prime Ministers as they leave office is hotly disputed, but few in modern British history have been subject to so much departing hostility as Tony Blair.

Anthony Seldon said Mr Blair had high ambitions for public services

Parts of the British media give the impression of a man who had no clear idea why he wanted to be the prime minister, who led the country into a senseless war in Iraq, and lied in doing so, who has kow-towed to not one but two American presidents, and whose record domestically has been full of lies, spin, empty rhetoric and unfulfilled promises.

Yet Tony Blair's successive election victories alone guarantee that he will be remembered as Labour's most accomplished electoral leader.

Having succeeded John Smith as Labour Party leader in 1994, he spent three years preparing his party to be fit for office, and stripping it of "Old Labour" policies on defence, trade unions and the economy.

The party came to power in May 1997 on the crest of a wave of popular emotion which swept away John Major's discredited and exhausted Tories.

The party won a second landslide in 2001, and, in 2005 a third victory, albeit on a much reduced majority.

Economic strength

Underpinning these victories was Britain's powerful economic performance. Tony Blair was fortunate in this inheritance in 1997, and in having such a capable chancellor in Gordon Brown, but good leaders to some extent create their own luck.

At home, Blair had high ambitions for building world-class public services, transforming the constitution, reducing inequality and attacking lawlessness.

But in the beginning progress was painfully slow. In his first term (1997-2001), Blair focused on reforming the constitution and introducing welfare reforms, which paved the way for reductions in pensioner and child poverty.

Blair prepared his party for three years before coming to power

But the former owed more to the legacy of his predecessor, John Smith, while the latter was the achievement principally of Gordon Brown.

For Blair personally, his first term was a learning experience, when mistakes were made and valuable time squandered.

Only in the second and third terms did his personal ideas fructify into his "choice and diversity" agenda, extending Margaret Thatcher's liberal reforms into the welfare state.

But by the time he began to introduce these policies, including top-up fees for universities, foundation hospitals and city academies, his political capital was waning and he had to push his agenda through in the face of opposition from his own party.

In his third term (2005-2007), shorn of the need to win another general election, he extended his policies into new areas, including nuclear energy, welfare, pensions reform, road pricing and greater independence for schools and health.

Fighting off the successive Brown challenges were essential for a prime minister who, unlike Mrs Thatcher, was embedding his personal agenda so late.

Northern Ireland push

The popular view is that Blair's first term was his most successful, and the second and third terms then became progressively worse. In fact, the very opposite is the case.

To no area of British life did Tony Blair give more consistent attention as prime minister than the affairs of Northern Ireland.

From his earliest weeks in power he decided that completing the peace process would remain a priority and his hard work reaped dividends through the Good Friday Agreement of 1998.

Signing the Good Friday peace agreement was a priority for Blair

He never lost sight of the need to give attention to the province and the fact that it has been largely peaceful during his 10 years owes much to his own efforts and determination.

In Europe he was less successful. He was passionate about building a qualitatively new relationship between Britain and the European Union.

But in his early months in power he was outflanked by Gordon Brown and the Treasury over taking Britain into the Euro.

His best chance gone, he was unable thereafter to take Britain into the single currency, or to take part in the new constitution, which foundered in 2005.

European role

Many commentators thought both failures a blessing for Britain. Blair did, however, play a pivotal role in extending the membership of the EU from 15 to 27 states, and toiled hard to ensure free-market rather than dirigiste and federalist values prevailed.

Blair's European policy was hampered by the hostility, often vitriolic, of Jacques Chirac and Gerhard Schroeder.

But Blair's own lack of long-term thinking on the EU, or full follow-through to his seminal speech to the European Parliament in June 2005, in which he proposed a bold new vision for the EU, also was responsible.

Abroad, his "Gladstonian" or "Wilsonian" foreign policy inspired his major speech in Chicago in April 1999 in which he articulated the doctrine of the "international community".

Blair still believes that it was right for Britain to support the Americans over Iraq, to remove a brutal dictator who he considered a menace to his own people and threat to world peace.

This advocated humanitarian intervention in the affairs of countries under tyrannical regime under certain conditions.

Blair used this approach to justify his intervention in the Kosovo crisis in 1999, when he urged a more aggressive military approach upon a reluctant American administration.

It also, in Blair's mind, provided much of the intellectual justification for the war against Iraq in 2003.

Iraq remains the most controversial aspect of Tony Blair's premiership.

Blair still believes that it was right for Britain to support the Americans over Iraq, to remove a brutal dictator who he considered a menace to his own people and threat to world peace.

Serious mistakes

Yet even loyal Blairites must now concede that serious mistakes were made: in the overselling of the intelligence, in the woefully inadequate planning for the postwar situation (with the US chiefly culpable) and in not doing more to rein in the excesses and inconsistencies of the Bush administration.

Tony Blair has always believed that Britain's interests were best served by standing shoulder to shoulder with the American administration, despite the resulting unpopularity at home and in Europe, and that any criticism of Washington should be made in private rather than in public.

The Iraq war sacrificed some of Tony Blair's domestic authority, his time, and ultimately detracted from his efforts to bring peace in the Arab Israeli conflict, to do more to eliminate poverty in Africa and to combat global warming.

All three were cherished causes to him. Again, starting work earlier in these areas would have allowed Blair to achieve much more.

That fact notwithstanding, few doubt that Britain's standing in the world, and in Europe, is higher in 2007 than in 1997.

All prime ministers are limited by external forces: in Tony Blair's case it was the erosion of support due to the Iraq war, the unremitting hostility to much of his reform agenda from his chancellor, Gordon Brown, a lack of Blairite ministers of quality to run the departments of state, or supporters in the party at large.

At his best, as in his response to the death of Princess Diana in 1997, to 9/11, or to the London bombings in July 2005, he was able to find a language that chimed with the national mood that few prime ministers, or indeed US presidents, have matched.

But the obstacles he encountered, his lack of governing experience, and his lateness in discovering his personal agenda, meant his achievements were less than he hoped for, or promised.

Prime ministers leave amid the noise of the immediate, in Blair's case the cash-for-honours scandal, anger over Iraq, turbulence over his succession and rows over the future direction of policy.

But, as long as a transition is free of scandal, history judges premierships in a more dispassionate way.

No-one can know for sure what his successors will do - the key factor which forges historical judgments.

But it is very unlikely that Blair's successors will revert to Old Labour, or that David Cameron will not build substantially on what Blair has achieved in power since 1997.